Yes, an email must be sent to them with a link, asking them to write a feedback. There can be rating from 0 to 5, and an optional feedback can be also given. Also, even when people use their client area and visit the ticket that is closed, normally the tickets are shown in a list, so you can add a button or something on the right side of it to rate the ticket.(That button can redirect to that place to rate and provide a feedback) This button would not show at that place after it is rated.